The present invention is directed to the field of medical imaging and, more particularly, to methods and systems for reducing background artifacts during single photon emission computer tomography applications and the like.
Without limiting the scope of the invention, its background is described in connection with single photon emission computed tomography (SPECT), as an example.
The subject of medical imaging covers the interaction of all forms of radiation with human tissue and the development of the technology to receive useful information from observations of this interaction. The information obtained from the interaction between tissue and radiation is usually represented in the form of two- or three-dimensional images. Medical imaging has wide application in diagnostic medicine as well as for monitoring the treatment of disease.
Single photon emission computed tomography (SPECT) generally includes the detection of radiation emanating from inside the human body. Internally emitted photons arise from the decay of radioactive isotopes commonly called radionuclides. The radiopharmaceutical (i.e., pharmaceutical labeled with a specific radionuclide) is generally injected into the patient and localizes within one or more organs based on its biochemical properties. Hence, particular radiopharmaceuticals are used to illuminate specific organs.
SPECT has been used extensively to produce images of tumors within the body. SPECT uses detectors to register gamma ray photons emitted from radioactive isotopes injected into the body. Although the detection of these photons is not difficult, the location of their origin is challenging. Contrary to visible light, gamma ray photons cannot be focused by lenses to convey the location of their source, hence little directional information can be obtained from the detection of these high-energy photons. The only way to locate their origin has been to use a collimator to remove photons from unwanted directions.
A typical SPECT system includes one or more rotating scintillation cameras fitted with collimators. This collimator-detector system has significant effects on the quality of the images. In typical SPECT, a lead collimator is placed in front of the crystal to selectively remove photons from all directions except the one of interest. The collimator is typically about 2 or 3 cm in thickness. Collimators generally include a number of small channels (holes) that allow photons traveling within the desired acceptance angle to pass through and be registered by a detector. There are many different types of collimators used in SPECT, which differ in the number and the direction of orientation of the channels. These types of collimators have the fundamental problem that the only way to obtain better directional information is to reduce the acceptance angle of a channel. Unfortunately, this reduction in angle is accompanied by a significant reduction in the percentage of incident photons which are passed through the collimator. The collimator design is, therefore, a trade off between good resolution and adequate sensitivity.
Collimators designed using coded aperture arrays have been used in astrophysics to detect gamma rays (Fenimore and Cannon, 1978, Dunphy, et al., 1988). A coded aperture array collimator consists of many small holes (commonly referred to as apertures) that are arranged randomly along a flat plate. Each point on the emitting object deposits a shadow of the aperture on the detector. Computer processing of the picture yields a reconstructed image of the original object. They have the advantage that good resolution may be achieved without sacrificing sensitivity. These collimators have been suggested for medical applications (Fenimore and Cannon; 1979), but have not achieved much success.
The present invention is directed to the field of medical imaging and, more particularly, to methods and systems for reducing background artifacts during single photon emission computer tomography using coded aperture array collimators applications and the like.
An example of an application of this invention, as described hereinbelow, is in the area of breast tumors. During mammography exams, the primary diagnostic tool for detecting breast cancer is an x-ray mammograph. Patients with large amounts of glandular breast tissue are susceptible to false negative mammograms using standard mammography techniques. Presently, alternative techniques using SPECT have limited resolution and an improved method of detecting small lumps is needed.
The coded aperture array collimators have the advantage of providing good resolution, but previous designs have suffered from background artifacts. It would, therefore, be desirable to reduce the significance of the reconstructed artifacts during SPECT imaging. In accordance with the present invention, a medical imaging system and method is provided that creates an accurate composite image by adding two images together, thereby causing a significant fraction of the background artifacts to be removed. Substantial reduction of background artifacts is achieved by the present invention through the employment of a novel collimator design and method of its use.
In accordance with an aspect of the present invention, a collimator is provided that includes a positive apertured portion and a negative aperture portion. This type of composite collimator has been suggested previously to improve the image of stars in astrophysics. It has not, however, been used for reducing background artifacts in medical imaging. The positive aperture portion is used in medical imaging systems to receive a first photon-generated image from a photon source (e.g., the breast of a human subject). The negative aperture portion is used to receive a second photon-generated image from the photon source. Both images are received by a detector and are used to derive a more accurate representation of the photon source by removing noise and artifacts from the image.
In accordance with another aspect of the present invention, a collimating system is provided in which a collimator has positive and negative aperture sections, the collimator being movable within a housing having a slot formed therein for storing and promoting the independent exposure of either the positive or negative aperture sections to a photon energy source during imaging operations. A housing having a slot retains the collimator. A detector receives photon energy through the aperture section of the collimator.
In accordance with another aspect of the present invention, a method of photon emission computing tomography is provided in which a first image of a targeted photon source is generated through a collimator having a positive aperture arrangement. A second image of the targeted photon source is then generated through a collimator having a negative aperture arrangement. Finally, the first and second generated images are summed, thereby causing a significant fraction of background artifacts to be reduced from the resulting summed representation of the targeted photon source.